This invention relates to a novel method of precipitating alkaline-earth metal titanates in powder form from clear, homogeneous solutions of the desired titanate constituents. More specifically, this novel method involves the direct precipitation of titanates from such solutions rapidly, and without the need for the application of any external heating source.
Barium titanate is a material of great interest for electronic applications due to its ferroelectric behavior, i.e., a spontaneous alignment of electric dipoles within the material itself. Modifying the transition temperature at which this ferroelectric behavior occurs allows the production of components such as multilayer ceramic capacitors and thermistors with optimized electronic properties. Materials such as other alkaline-earth titanates and zirconates are widely known as being effective in performing such modifications.
As the trend in electronic circuitry continues toward higher levels of sophistication, increased board densities, and increased volumetric efficiencies in components, the need to supply newer, improved materials for component production is emerging. Conventionally, barium titanate and other titanate materials are prepared by high temperature calcination of the appropriate precursor materials. Specifically, barium carbonate (BaCO.sub.3) and titanium dioxide (TiO.sub.2) are mixed in the desired stoichiometric amounts and calcined at 1000.degree.-1200.degree. C. to form barium titanate (BaTiO.sub.3). Materials produced in this manner are hindered in their applications for the smaller, more sophisticated devices for several reasons. Mixing and post-calcination pulverization processes introduce uncontrollable and undesirable levels of impurities. The high calcination temperatures yield a powder which is of large and non-uniform grain sizes. In addition, the calcining process is expensive and thermal treatment is difficult to control. Also, mixing and calcining of precursors in the solid form is not entirely homogeneous and variations in particle to particle stoichiometry certainly exist.
Another popular method for commercial production of barium titanate ceramic powders is the thermal decomposition of barium titanyl oxalate tetrahydrate BaTiO(C.sub.2 O.sub.4)2.4H.sub.2 O to form very fine BaCO.sub.3 and TiO.sub.2 crystallites which subsequently react in the solid state to form BaTiO.sub.3. (Clabaugh, W. Stanley, Swiggard, Edward M., and Gilchrist Raleigh, "Preparation of Barium Titanyl Oxalate Tetrahydrate for Conversion to Barium Titanate of High Purity", Journal of Research of the National Bureau of Standards, Vol. 56, No. 5, May 1956, pp. 289-291). Although this method developed by Clabaugh et al. produces a titanate material of higher purity and reactivity than the mixed oxide calcination process, high pyrolization/calcination temperatures of 750.degree.-1100.degree. C. are still required to form single phase BaTiO.sub.3. In addition, Ba.sup.++ ion solubility in the precipitate mother liquor and the solid-state reaction processes necessary to form BaTiO.sub.3 do not allow a resultant material which is completely homogeneous in stoichiometry.
Various other methods such as those disclosed in U.S. Pats. Nos. 3,330,697 and 4,534,956, as well as others such as sol-gel or alkoxide methods have also been successfully employed. (Ritter, J. J., Roth, R. S., and Blendell, J. E., "Alkoxide Precursor Synthesis and Characterization of Phases in the Barium-Titanium Oxide System", Journal of the American Ceramic Society, Vol. 69, No. 2, 1986. pp. 155-162; Mazdiyasni K. S., Dolloff, R. T., and Smith J. S. II, "Preparation of High-Purity Submicron Barium Titanate Powders", Journal of the American Ceramic Society, Vol. 52, No. 10, 1969, pp. 523-526; and Wu, Edward, Chen, K. C., and Mackenzie J. D., "Ferroelectric Ceramics--The Sol-Gel Method Versus Conventional Processing", Materials research Society Symposia Proceedings, Vol. 32, Better Ceramics Through Chemistry, Copyright 1984 by Elsevier Science Publishing Co, Inc. pp. 169-174). Although these methods result in fine sub-micron powders of near uniform size, they are also hindered by such factors as the need for calcination treatments, exotic manufacturing schemes, low product yields, and/or the use of exotic precursor materials.
The literature has also suggested techniques for the hydrolysis of titanium esters in the presence of alkalineearth metal ions at higher pH values as a route to BaTiO.sub.3 formation. (Flachen, Steward S., "An Aqueous Synthesis of Barium Titanate", Journal of The American Chemical Society, Vol. 77, 1955, p. 6194; and Kiss, Klara, Mager, Jules, Vukasovich, Mark S., and Lockhart, Robert J., "Ferroelectrics of Ultra-fine Particle Size: I, Synthesis of Titanate Powders of Ultra-fine Particle Size", Journal of The American Ceramic Society, Vol. 49, No. 6, 1966, pp. 291-295.). However, sophisticated laboratory apparatus and external heating sources were employed to develop critical conditions necessary to form the desired product. The degree of control that is necessary would most certainly limit the commercialization potential for these processes.
The recently issued U.S. Pat. No. 4,520,004 discloses a process for manufacturing fine alkaline-earth metal titanates by combining a water soluble salt of Ba, Sr, or Ca with a hydrolized product of a titanium compound in an aqueous alkaline solution having a pH greater than 13. More specifically, this method begins with the preparation of an inorganic titanium compound such as TiO.sub.2.xH.sub.2 O by neutralizing TiCl.sub.4 or Ti(SO.sub.4).sub.2 in an aqueous or alkaline solution, and then reacting the product with a water soluble salt of Ba, Sr, or Ca in an aqueous alkaline solution having a pH of 13 or more and a temperature of approximately 100.degree. C. The process produces a fine precipitate which can then be filtered from the solution. The disadvantage of this process is that, as a practical matter, it requires the application of an external heating source to maintain elevated temperature during the reaction of the hydrolysis product with the water soluble salt. The temperatures necessary for conversion are selected to be preferably above 60.degree. C., and are actually in the 100.degree. C. area to optimize the process.
Contrary to the above-noted teachings, applicants have discovered that metal titanates, and in particular alkaline-earth metal titanates and combinations thereof, can be directly synthesized or precipitated from a complex titanium alkoxide immersed in an acetic acid solution without requiring the application of any external heating source during the precipitation process (i.e., relying solely on the heat of chemical reaction to facilitate BaTiO.sub.3 production) and without requiring the use of a water soluble alkaline earth metal salt.
A primary object of this invention, therefore, is to provide an improved method of precipitating fine metal titanates of precise stoichiometric proportions directly from a mixture of a complex titanium alkoxide and an alkaline earth metal, without having the need to apply any external heating source to enact the precipitation process.
Still another object of this invention is to provide an improved method of the type described which obviates the need for utilizing the more costly and less commercially-available water soluble alkaline earth metal salt as a precursor in the process.
Other objects of the invention will be apparent herein to one skilled in the art from the specification and from the appended claims, particularly when read in conjunction with the accompanying drawings.